The present invention relates to a lens system that is used for an optical apparatus such as a digital still camera.
As a focusing method for a high zoom ratio optical system, a front lens feed method for feeding a lens group disposed closest to the object (e.g. see Japanese Laid-Open Patent Publication No. H11-258504) and an internal focusing method (e.g. see Japanese Laid-Open Patent Publication No. 2004-212612) have been known.
However if focusing is attempted using the conventional front lens feed method, the support mechanism and driver mechanism of the focusing lens group tend to be large, since the large and heavy lens group that is disposed closest to the object is normally moved. The total length of the lens system also tends to increase upon focusing on an object at close distance.
If the conventional internal focusing method is used, an advantage is that the support mechanism and drive mechanism of the focusing lens group can be compact, since the focusing lens group is a second or subsequent lens group, which is lighter than the first lens group disposed closest to the object. However in the case of the internal focusing method, the focusing mechanism tends to become complicated, since focusing cannot be performed on objects at a same photographic distance with a same feed amount throughout the entire zooming range from the wide angle end state to the telephoto end state.
Further, in order to prevent a photographic error due to an image blur caused by hand motion, it is desired that the above mentioned high zoom ratio zoom lens has an image blur correction function, which corrects an image blur on the image plane by setting all or a part of one lens group, out of the lens group constituting the lens system, as a shift lens group, and shifting the shift lens group so as to have a component approximately orthogonal to the optical axis, according to a value that is output by a detection system for detecting a blur of the lens system. Generally for a shift lens group, it is preferable to select a lens group located near a diaphragm where the abaxial flux of light passes near the optical axis upon zooming, so as to minimize the performance deterioration during lens shift.
Moreover many optical systems with high zoom ratios have a vibration proof function for correcting an image blur on an image plane by decentering all or a part of one lens group, out of the lens groups constituting the lens system, as a shift lens group, in order to prevent photographic errors due to an image blur caused by hand motion. However if a lens group which moves during zooming, is decentered for the purpose of vibration proofing as in the case of a conventional optical system, the optical performance may dramatically drop, which makes it impossible to obtain good images.